1. Field of the Invention
The present invention relates to a method for producing a polyolefin composition containing an additive or additives.
2. Description of the Related Art
In order to prevent thermal degradation and oxidation degradation in a polyolefin, additives such as a phenol-based antioxidant and/or calcium stearate are added thereto and then a polyolefin is used in the form of a polyolefin composition.
Generally, when a powdery additive is added to a polyolefin, there are problems with dusting and dust explosibility of the additive. As a method to solve these problems, Japanese Unexamined Patent Publication (Kokai) No. 8-333477 (Example 2 in paragraph [0142] and Table 2 in paragraph [0144]) proposes adding a particulate additive composition (granules) to a polyolefin. Specifically, a particulate additive composition having a maximum length of 2 to 4.5 mm formed by heating a mixture of a phenol-based antioxidant and calcium stearate in a twin screw extruder to 205° C. and then extruding the mixture from holes having a diameter of 2 mm is disclosed.
A particulate polyolefin having a size of 250 μm or less before addition of an additive is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 10-53679 (Table 1 in paragraph [0021]). Also, a polyolefin having a weight average particle diameter of 390 to 610 μm before addition of an additive is specifically disclosed in the Examples in Japanese Unexamined Patent Publication (Kokai) No. 2000-143706 (Examples).
There is nothing specifically disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-333477 about adding a particulate composition (A) containing an additive or additives to a particulate polyolefin (B). Hypothetically, when the particle diameter (a) of the particulate composition (A) is 2 mm and the particle diameter (b) of the particulate polyolefin (B) is 250 μm, there is added at a particle diameter ratio (a/b) of about 8, and when the particle diameter (a) of the particulate composition (A) is 2 mm and the particle diameter (b) of the particulate polyolefin (B) is 610 μm, there is added at a particle diameter ratio (a/b) of about 3.1.
The present inventors, upon examination of a polyolefin composition obtained by adding a particulate polyolefin (B) having an average particle diameter so that the particle diameter ratio (a/b) is 5.2 to a particulate composition (A) containing a compound represented by formula (1), which is one type of phenol-based antioxidant, and calcium stearate, found the problems of large variation in MFR and of insufficient dispersion of the additive in the polyolefin.
wherein R1 represents an alkyl group having 1 to 8 carbon atoms, X represents an n-hydric alcohol residue having 1 to 18 carbon atoms which may include a heteroatom and/or a cyclic group, and n is the integer 2 or 4.